The invention relates to a method of transmitting data through an isochronous channel, and applies in particular but nonlimitingly to networks based on the IEEE 1394 standard.
In contradistinction to isochronous transfer, the transfer of files (namely any information not presented in the form of a “stream”) over a network based on one or more IEEE 1394 type serial buses is done asynchronously. Specifically, the asynchronous route makes it possible to cater for an acknowledgement of receipt mechanism, which does not exist in isochronous mode. This mechanism makes it possible to request a new transmission of corrupted packets, guaranteeing the reliability of the transmitted data.
In the least favourable case, the asynchronous route uses only 20% of the global bandwidth, the remainder being reserved for the isochronous route. This leads to an overly long transmission time when transferring voluminous files.
It would therefore be desirable to be able to transfer files isochronously, with the aim of being able to profit from the wider bandwidth reserved for this mode. The absence of guarantee of proper reception of the transmitted packets does not however allow the use of the isochronous mode for the reliable transmission of data. Packets may therefore be irredeemably lost, making all the data transmitted unusable, for example in the case of the transmission of the code of an application. For isochronous streams of audio or video type, the loss of one or more packets is of hardly any importance: on the one hand mechanisms for masking such errors generally exist, and on the other hand, the streams comprise, at relatively short intervals, entry points requiring no knowledge of the past stream. For files or other data which have to be transmitted in full and without error, this loss of packets is obviously not acceptable.
In addition to the corrupting of packets in the steady state, it may happen that several hundred kilo-bytes are missing at the start of an isochronous stream. At file level, this would result in the deletion of the header, this being equally unacceptable.